Characteristics of spatial protein expression in the mouse cochlear sensory epithelia: Implications for age-related hearing loss

Hair cells in the cochlear sensory epithelia serve as mechanosensory receptors, converting sound into neuronal signals. The basal sensory epithelia are responsible for transducing high-frequency sounds, while the apex handles low-frequency sounds. Age-related hearing loss predominantly affects hearing at high frequencies and is indicative of damage to the basal sensory epithelia. However, the precise mechanism underlying this site-selective injury remains unclear. In this study, we employed a microscale proteomics approach to examine and compare protein expression in different regions of the cochlear sensory epithelia (upper half and lower half) in 1.5-month-old (normal hearing) and 6-month-old (severe high-frequency hearing loss without hair cell loss) C57BL/6J mice. A total of 2,386 proteins were detected, and no significant differences in protein expression were detected in the upper half of the cochlear sensory epithelia between the two age groups. The expression of 20 proteins in the lower half of the cochlear sensory epithelia significantly differed between the two age groups (e.g., MATN1, MATN4, and AQP1). Moreover, there were 311 and 226 differentially expressed proteins between the upper and lower halves of the cochlear sensory epithelia in 1.5-month-old and 6-month-old mice, respectively. The expression levels of selected proteins were validated by Western blotting. These findings suggest that the spatial differences in protein expression within the cochlear sensory epithelia may play a role in determining the susceptibility of cells at different sites of the cochlea to age-related damage.

Human pluripotent stem cell-derived inner ear organoids recapitulate otic development in vitro

Our molecular understanding of the early stages of human inner ear development has been limited by the difficulty in accessing fetal samples at early gestational stages. As an alternative, previous studies have shown that inner ear morphogenesis can be partially recapitulated using induced pluripotent stem cells directed to differentiate into inner ear organoids (IEOs). Once validated and benchmarked, these systems could represent unique tools to complement and refine our understanding of human otic differentiation and model developmental defects. Here, we provide the first direct comparisons of the early human embryonic otocyst and fetal sensory organs with human IEOs. We use multiplexed immunostaining and single-cell RNA-sequencing to characterize IEOs at three key developmental steps, providing a new and unique signature of in vitro-derived otic placode, epithelium, neuroblasts and sensory epithelia. In parallel, we evaluate the expression and localization of crucial markers at these equivalent stages in human embryos. Together, our data indicate that the current state-of-the-art protocol enables the specification of bona fide otic tissue, supporting the further application of IEOs to inform inner ear biology and disease.

Morphogenesis of neurons and glia within an epithelium

To sense the outside world, some neurons protrude across epithelia, the cellular barriers that line every surface of our bodies. To study the morphogenesis of such neurons, we examined the C. elegans amphid, in which dendrites protrude through a glial channel at the nose. During development, amphid dendrites extend by attaching to the nose via DYF-7, a type of protein typically found in epithelial apical ECM. Here, we show that amphid neurons and glia exhibit epithelial properties, including tight junctions and apical-basal polarity, and develop in a manner resembling other epithelia. We find that DYF-7 is a fibril-forming apical ECM component that promotes formation of the tube-shaped glial channel, reminiscent of roles for apical ECM in other narrow epithelial tubes. We also identify a requirement for FRM-2, a homolog of EPBL15/moe/Yurt that promotes epithelial integrity in other systems. Finally, we show that other environmentally exposed neurons share a requirement for DYF-7. Together, our results suggest that these neurons and glia can be viewed as part of an epithelium continuous with the skin, and are shaped by mechanisms shared with other epithelia.

sox2 and sox3 Play unique roles in development of hair cells and neurons in the zebrafish inner ear

Formation of neural and sensory progenitors in the inner ear requires Sox2 in mammals, and in other species is thought to rely on both Sox2 and Sox3. How Sox2 and/or Sox3 promote different fates is poorly understood. Our mutant analysis in zebrafish showed that sox2 is uniquely required for sensory development while sox3 is uniquely required for neurogenesis. Moderate misexpression of sox2 during placodal stages led to development of otic vesicles with expanded sensory and reduced neurogenic domains. However, high-level misexpression of sox2 or sox3 expanded both sensory and neurogenic domains to fill the medial and lateral halves of the otic vesicle, respectively. Disruption of medial factor pax2a eliminated the ability of sox2/3 misexpression to expand sensory but not neurogenic domains. Additionally, mild misexpression of fgf8 during placodal development was sufficient to specifically expand the zone of prosensory competence. Later, cross-repression between atoh1a and neurog1 helps maintain the sensory-neural boundary, but unlike mouse this does not require Notch activity. Together, these data show that sox2 and sox3 exhibit intrinsic differences in promoting sensory vs. neural competence, but at high levels these factors can mimic each other to enhance both states. Regional cofactors like pax2a and fgf8 also modify sox2/3 functions.